The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Research on realizing more shift-stages of an automatic transmission are undertaken to achieve enhancement of fuel consumption and better drivability.
In the field of the automatic transmission, a structure of the multistage transmission having the excellent linearity of the interstage ratio relates to a drivability such as an acceleration before and after the gear shift as well as a rhythm feeling of the engine speed.
However, as the number of shift stages is increased, the number of parts of the automatic transmission is increased, and as a result mountability, cost, weight, delivery efficiency, and the like may be rather aggravated.
As an effort to increase the fuel efficiency by multi-staging with reduced number of parts, an eight-speed automatic transmission has been introduced recently and a planetary gear train for an automatic transmission enabling more shift-stages has been investigated.
However, in the case of the recent eight-speed automatic transmission, we have discovered that because a span of a gear shift ratio (a factor for providing the linearity of the ratios between gear shift stages) is maintained at a level of 6.5 to 7.5, a power performance and a fuel consumption with a downsized engine is below a desired level of efficiency.
Also, in the eight-speed automatic transmission, since it is disadvantage to provide the linearity of the ratios between gear shift stages when the span of the gear shift ratio rises a level above 9.0, the operation efficiency of the engine and the drivability of the vehicle are negatively affected such that the development of automatic transmission of at least nine-speed or more is desired for its efficiency.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art.